In order to obtain insights into the mechanisms by which the cells in primary visual cortex generate their receptive field properties, we have investigated the intrinsic cortical circuitry by intracellular recording and dye injection. The dye injection technique in combination with 3-dimensional computer graphic reconstructions from serial sections allows us to visualize the full dendritic and axonal arbors of cells. We have also studied cells that are postsynaptic to the injected cells by serial EM reconstruction of the postsynaptic dendrites. Taken together, this methodology has extended our knowledge of interlaminar and horizontal cortical connections, and we present hypotheses on the relationship between these connections and specific receptive field features.